Currently most broadband orthogonal frequency division multiple access (OFDMA) wireless access technologies are designed to support high-speed packet data traffic. Accordingly, these technologies typically allocate high-rate data subchannels designed to support non-real time high-speed packet data traffic. Although many operators would like to exploit these technologies by offering real-time, low-rate, voice-based services, there are a number of unresolved issues related to the use of high-speed channels for low-rate voice services.
One issue is the low granularity of data channels. This results in waste of symbols for low-rate traffic and a shortage in number of traffic channels when large number of mobile stations are to be supported. Specifically, in scalable OFDMA systems where multiple Fast Fourier Transform (FFT) sizes are defined, but the subcarrier spacing remains constant, the number of definable traffic subchannels may be very small for lower FFT sizes. For example, when the channel bandwidth is 1.25 MHz there are only two subchannels per symbol. Accordingly, only a few bursts can be supported per frame.
Another issue is achieving low latency transmission for delay sensitive signaling and voice traffic. Low latency requires mobile stations to have transmission opportunities in almost every frame. However, this reduces the number of supportable active mobile stations.
Using low-rate voice and signaling channels in a make-before-break soft handoff, needed for delay sensitive traffic, causes challenges associated with synchronized scheduling of channels by two base stations involved in the soft handoff, and the interference management when frequency reuse factor is one.
One technology that uses OFDMA is the IEEE 802.16e standard. Except in broadcast or multicast scenarios, and contention-based ranging, the current OFDMA subchannlization defined in the IEEE 802.16e standard does not allow two or more users to share the same data channel on a time division basis. Accordingly, the current OFDMA subchannelization defined in IEEE 802.16e lacks the flexibility to provide a large number of low-rate traffic channels (or large number of mobile stations receiving low-rate signaling traffic channels), especially those which are close to the base station.